This invention relates to the field of DC to AC inverters, particularly those producing power at a high voltage sinusoidal waveform for powering equipment requiring high voltage AC power at frequencies or voltages that are independent of the frequency or voltage of a conventional AC power source.
In the past, systems to convert DC to high voltage AC suffered from various shortcomings. The available AC output voltage from inverters was generally limited by the voltage rating of available power semiconductors in the converter. In the past, operating power semiconductors in series had not been seen as a practical solution because of the difficulty in achieving simultaneous switching of series sets. Nevertheless, in the present invention, several methods are described for operating a number of AC power stages in series wherein the output AC voltage can be achieved at arbitrarily high levels.
The need for such inverters is usually in the operation of high power motors. It is common for such motors to require input voltages in the range of 4.16 KVAC to 13 KVAC. If a voltage of 4.16 KVAC is to be generated in a single stage, and if the power semiconductors are selected to have a conservative 50% voltage margin, then the DC source voltage must be approximately 3600 VDC and the semiconductors would require a rating of 5400 Peak volts. In order to deliver controlled AC voltage with low harmonics to an AC load, it is generally a requirement that the switching speed be high. However, high voltage devices tend to have high switching losses and so it is not practical to use high voltage power semiconductors to control high AC output voltages in systems requiring high repetition rates for the switching devices.
One alternative is to use a plurality of series connected pulse width modulation (PWM) inverters to achieve high AC output voltages. However this alternative is relatively expensive and complicated.
The present invention achieves high frequency switching and use of high voltage semiconductors without incurring high switching power losses. The present invention utilizes a pair of step wave generators in conjunction with a single PWM inverter in series connection to generate each phase of the AC output voltage. Since the step wave generators are operated at a relatively low frequency (low repetition rate), high voltage devices are readily available for such duty. A single set of relatively low voltage power semiconductors are operated in PWM mode (at a relatively higher frequency) to “fill in” the steps in the waveform generated by the step-wise output of the step wave generators or inverters.